In a communication system suitable for voice calls, a callee's presence (or status) can typically be known by the caller after a voice call is made. For example, if the dial-tone is busy, the callee is on the phone. A caller may also determine the presence of a callee by monitoring the callee's busy lamp. For example, if the callee's busy lamp signals red, the callee is likely to be on the phone and therefore not available to take an incoming call.
A voice call can be made by a variety of communication devices. Example communication devices may include desk phones, cellular phones, softphones, and smartphones. Different communication devices can use different communication services for communications. For example, a desk phone may use a fixed-line phone service to communicate through a public switched telephone network (PSTN), a cell phone or a smartphone may use a mobile telephony service to communicate through a cellular network, and a softphone may use a broadband service to communicate though a broadband network. In some cases, a voice service user may use more than one communication device. Since the communication services used by different communication devices are often distributed rather than centralized, a unified presence of the voice service user may not be available to other users in the communication system.